An Overview of Incoherent
Scatter Radar Systems
Ian McCrea
RADAR
• acronym for
RAdio Detection And Ranging:
• Generic name for measuring systems deriving
information about distant objects (radar targets) by
illuminating them with RF energy and recording the
reflected and/or back-scattered energy,
• In the early days, only the presence of, and range to, the
radar targets could be inferred, (that is how the acronym
became RADAR...)
• Today, many radar systems also allow spectral analysis
and (sometimes) target imaging.
A generic radar system
Radar target:

Physics
Engineering
Transmitting antenna:
TX
Signal
generator
Receiving antenna:
GT
Power:
P
Timing & Control
RX
A/D
Ar
To
computer
An Idealised Radar System
env(t)
e(t)
t
t’
z(t)z(t’)
Lag Profiles
Correlator
p(t)
Z=(p*e)
Receiver
How do Incoherent Scatter Radars Work?
We’re going to look at:
• Klystrons
•Antennas
•The Radar Equation
•Raw Electron Density &
System Constant
•Sampling
•Frequency Mixing
•Filtering
•The Range-Time Diagram
•Correlation
•Cross Products
•The Lag Profile Matrix
•Gating
•Ambiguity Functions
•Requirements of Experiments
•Signal, Background and Calibration
•Clutter
•ACFs and Lag Profile
How a klystron works
modulator
R/C
Beam ctrl
voltage
Uin
Uout
cathode
collector
mod anode
UCin
ve
Lp
C
-
+ U(t)
Electrons are emitted from the cathode and form a space-charge cloud around it. When the beam control electrode (”mod-anode”) is raised
to a high positive DC voltage Um, the electrons are accelerated. They pass through the mod-anode and are focussed into a beam, which is
further accelerated by a voltage U. The beam goes through a series of resonant RF cavities to a collector. The beam current does not reach
its full value instantaneously; the parasitic inductance of the the supply leads L p introduces a time constant, which in the UHF is  10 s.
A RF signal applied to the first cavity sets up a RF voltage UCin across the first cavity aperture, which density modulates the electron beam,
creating ”bunches” of charge at the RF rate; i.e. the electron beam current is RF modulated. Propagating down the drift tube, the beam
excites each cavity in turn and the induced fields amplify the density modulation. At the output cavity, the beam is almost fully bunched.
This cavity is connected to a load (the antenna) and the beam now gives up that part of its kinetic energy stored in the RF structure (over
half of the total) in the form of a strong RF current that drives the load. The power gain (input-output) can be over 50 dB. Leaving the
output cavity, a little less than half the initial kinetic energy is left in the beam and this is dumped into the collector as waste heat, which
must be removed by water cooling.
When not transmitting, the electron beam is shut down by applying a negative (ie repulsive) voltage (a few kV) to the mod-anode.
The mod-anode voltage is generated by a modulator; we can control the modulator by the BEAMON and BEAMOFF commands. Because of
the time delay between the mod-anode voltage and the beam current, we cannot start to transmit RF immediately after a BEAMON but
must first wait 3-4 time constants (30-40 s) for the beam current to stabilise.
Transmitted power is generated in a big electron tube known as a Klystron
Electron gun generates
a stream of electrons that
flow towards the collector
Electric field of RF modulates
the speed of the electron beam
concentrating it into bunches
A large fraction of the
electron beam energy is
dissipated at the collector
Collector
Cathode
The amplified RF is
extracted through a
waveguide
Modulating anode
RF fed into first
resonant cavity
Further resonant cavities
increase the bunching
and amplify the output
EISCAT’s klystrons cannot transmit
Continuously. They are limited to a
duty cycle of ;
12.5% on the mainland
25% on Svalbard.
Even if it were possible to transmit
continuously, breaks in transmission
are necessary in order to receive the
returned signal.
ESR Klystrons
EISCAT Antennas
• UHF antennas
–
–
–
–
32-meter fully steerable Cassegrain dishes
Wheel-on-track az drive, rack-and-pinion elevation drive
Max. rotation 1.5 turns in azimuth, 0-90o in elevation
Max. angular speed 1.2o/second both axes
• ESR 32-meter antenna
– Fully steerable Cassegrain dish
– Rack-and-pinion drive in az and el, angular speed up to 3o/second
– Max. rotation 1.5 turns in azimuth, 0-180o in elevation
• ESR 42-meter antenna
– Fixed Cassegrain
– Pointing along tangent to local field line @ 300 km
– Feed adjustable to follow the secular variation in field until > 2007
• VHF antenna
–
–
–
–
40 x 120 m parabolic trough
Can be run as two independent, electrically steerable arrays
Elevation range (15 – 90)o
Computer control presently disabled
EISCAT Svalbard Radar
Antennas and radiation patterns
The EISCAT UHF and ESR use parabolic Cassegrain reflector antennas.
To understand how they work, we recall that the shape of the far-field
radiation pattern of a uniformly illuminated circular reflector of diameter
DM, operating at wavelength  (the main reflector of a Cassegrain
antenna), is the same as that resulting from Fraunhofer diffraction of a
plane wave illuminating a circular aperture set into an infinite baffle
(Babinet’s principle).
The intensity of the diffracted field from the main reflector at an angle 
is S():
S() = S(0) [  DM / 2 sin ] 2 J1 (DM sin  / ) 2
where  is the angle between the direction of observation and the optical
axis, S(0) is the on-axis intensity and J1 is the first order Bessel function.
The Cassegrain optics of the EISCAT antennas also contains secondary
reflectors. These are used to illuminate the main reflectors, but at the
same time they also block part of the main reflector apertures.
ESR 42m (fixed) dish
Secondary reflector and feed – ESR 42m dish
Antennas and radiation patterns
The subreflector blockage can be modelled as diffraction from a circular
obstacle of diameter = Ds (the subreflector diameter). The composite
diffraction pattern of the Cassegrain system then becomes Sc():
Sc() =
S(0) [ / sin ] 2 [DM2 – DS2]-2 •
• {[DM J1(DM  sin /)] 2 – [DS J1(DS  sin /)] 2}
Example:
EISCAT 32-meter UHF
DM = 32.0 m, DS = 4.58 m,  = 0.33 m
=> -3dB (theoretical) = 0.6 degrees
When DM >> DS, the full –3 dB
opening angle (FWHM) of the main
lobe of the diffraction pattern is
-3dB  0.89 /DM (radians)
Radiation patterns and transverse resolution
The real antenna pattern differs from the theoretical Sc for several reasons:
- the apertures are not uniformly illuminated (physically impossible!),
- the illumination does not taper off to zero at the reflector edges,
- the subreflector tripod struts shade the main reflector...
Measured pattern of EISCAT
Tromsø UHF antenna
-3dB (actual) = 0.7 degrees
The main beam opening angle determines
the transverse (cross-beam) resolution
As a rule of thumb, the actual –3dB
opening angle of a well designed
Cassegrain antenna is very close to
-3dB =  /D
At a distance R, this corresponds to a
transverse -3dB resolution of
rt = R 
For the EISCAT UHFat R = 100 km:
rt (100km) = 1.22 km
EISCAT Antennas: Aperture Area and Gain
A result from antenna theory:
G = 4 A/2
If the antenna aperture is circular with
diameter = D, the maximum gain is
Gmax = 2 D2/ 2
But:
The diameter of an EISCAT UHF
antenna is 32 m. Operating at 930 MHz,
the maximum gain becomes
1)
A large aperture area picks
up more scattered signal on
receive
Gmax(UHF) = 97260x or 49.88 dBi
2)
Higher gain translates into
better angular resolution
So when transmitting through this
antenna, the power density in the far
field is almost 105 times the isotropic
power densitybut at the same time we illuminate only
10-5 as many electrons, so the total
scattered power doesn’t increase !
ESR 32m (steerable) dish
ESR: Facts and Figures
Antennas: 32m (steerable), 42m (fixed)
Gain: 45dB (32m), 42.8 dB (42m)
Figure of Merit: 40.66 (42m), 24.5 (32m) MWm-2K-1
Frequency Range: 500 MHz ± 5 MHz
Transmitter Type: 16 modular “TV transmitter” type
Transmitter (Average) Power: 250 kW
Duty Cycle: 25%
Hours/Year: > 1000
Pulse Codes: Alternating, random codes, long pulses
Tromsø UHF Radar
UHF: Facts and Figures
Antennas: 32m (steerable) (all 3 sites)
Gain: 48.1 dB
Figure of Merit: 36.27 MWm2K-1
Frequency Range: 928 MHz ± 4 MHz
Transmitter type: (2x) Klystrons (Tromso only)
Transmitter (Average) Power: 163 kW
Duty Cycle: 12.5%
Hours/Year: > 1000
Pulse Codes: Alternating, random codes, long pulses
Kiruna UHF Radar
Monostatic and Multistatic Radars
•A monostatic radar has a co-located transmitter and receiver
•Monostatic radars probe the ionosphere much more efficientl
but the transmission must be pulsed
•The range resolution is defined by the pulse length
•We will see that there are many tricks for changing the
modulation of the transmitted signal in order to optimise the
range resolution of the radar.
Tx, Rx
•A multi-static radar may have many passive receivers
•The largest possible measuring volume is defined by the
intersection volume between the two radar beams.
•This is defined by the beamwidth in the transverse direction
Tx
Rx
•Multistatic radars are essential for determining plasma
velocity, electrodynamics, interplanetary scintillation etc.
Sodankylä UHF Radar
EISCAT Sodankylä Radar
EISCAT Tromsø VHF and UHF Radars
Tromsø VHF Radar
VHF: Facts and Figures
Antennas: 120 x 40m cylinder
Gain: 48.1dB
Figure of Merit: 36.62 MWm-2K-1
Frequency Range: 224 MHz ± 1.5 MHz
Transmitter Type: 2 (large) klystrons
Transmitter (Average) Power: 250 kW
Duty Cycle: 12.5%
Hours/Year: > 1000
Pulse Codes: Alternating, random codes, long pulses
Tromsø VHF Radar (split-beam mode)
Reflecting surface and dipole feeds – VHF antenna
Inside the
feeder bridge
of the Tromsø
VHF radar
The Radar Equation
Consider the most general case of a bistatic radar with separate transmitting and
receiving antennas.
Let the transmitting antenna have gain G1, and the receiving antenna have gain G2 and
effective aperture Ae2. Let’s assume we’re illuminating a volume whose position vector
from antenna 1 is r1, and from antenna 2 is r2.
Let’s assume our scattering volume contains only one electron!
The power at the receiver is:
G1 (r1 ) Ae 2 (r 2 )
Pr (r )  r Sin Pt
4r12
r22
2
0
2
or
2
G
(
r
)
G
(
r
)

1
2
Pr (r )  r02 Sin 2 Pt 1 2 2 2
4r1 4r2 4
or
G1 (r1 ) Ae 2 (r 2 )
Pr (r )   0 (  ) Pt
4r12 4r22
Assuming;
Scatterers uniformly fill
the scattering volume (a
“diffuse target”)
No multiple scattering
(the Born criterion)
The Radar Cross-Section
The quantity;
 0  4r sin   110
2
0
2
28
m sin 
2
2
is known as the radar cross-section (per electron). If a target electron scatters
radiation uniformly in all directions, then total scattered power is the incident
intensity times the radar cross-section, and a fraction d/4 would be scattered
into solid angle d.
In fact, this is an approximation. The full form is:
4r sin 
0 
Te
2 2
(1  k D )(1   k 2 2D )
Ti
2
0
2
Note the dependence on plasma temperature ratio and Debye length
Application to a Real Radar
Let’s plug in some realistic numbers to see the size of our problem:
•At range 300 km, the radar beam cross-section is about 106m2 (1 km x1 km).
•If the transmitted power is 1 MW, the incident intensity is 1 Wm-2.
•If the range resolution of our pulse is 10 km, the size of the illuminated volume is
1010m3.
•Now assume that the electron density, Ne, is 1012 m-3.
•The total radar cross-section is thus NeV0 ~ 1012 x 1010 x 10-28 ~ 10-6 m2.
•If the effective aperture of our antenna is 100 m2, then:
10 6 m 2  1W / m 2  100m 2
16
Pr 

10
W
2
2
4  300 km
So we need a radar which is not only sensitive enough to receive such small
powers, but which doesn’t swamp the received signal with thermal noise.
Getting the Electron Density from the Received Power
If we know the transmitted power, the size of the scattering volume and the
effective aperture of the dish, we can derive the electron density from the received
power.
For a monostatic radar, G1 = G2 = G and r1 = r2 = r. The power coming from a
small volume element V located at r is;
2
2
G
(
r
)

d 3 Pr (r )  ne (r )Pt
dV
2 2
(4r ) 4
In spherical coordinates, the range gate r long has a size dV=r2dr and so the
power received from the height interval r to r+r is;

ne (r )Pt 2  2
Pr (r ) 
G ()d r
3 2 
(4 ) r 

Getting the Electron Density from the Received Power
Inverting this equation,
Te
2 2
(
1

k

)(
1


k
D )
2
Ti
Pr (r ) r
ne ( r )  C
Pt r
4r02
2
2
D
C is often referred to as “the system constant”, since it is determined by the
antenna beam pattern, the maximum gain and the radar wavelength. It should
therefore be a constant for any given radar system. If we assume that Te = Ti,
and that k2D2 << 1, then the equation simplifies to
Pr (r ) r 2 2
ne (r )  C
Pt r 4r02
We often call this estimate the “raw electron density”.
The World’s Incoherent Scatter Radar Facilities
Arecibo Radar, Puerto Rico
Arecibo: Facts and Figures
Antenna: 304.8 m reflector (!!)
Gain: 58.5 dB
Figure of Merit: 344.6 MWm2K-1
Frequency Range: 430 MHz ± 500 kHz
Transmitter Type: Klystrons
Transmitter (Average) Power: 150 kW
Duty Cycle: 6%
Hours/Year: ~1200 (15% of operations)
Pulse Codes: Alternating, random codes, long pulses
Sondre Stromfjørd IS Radar, Greenland
Sondy: Facts and Figures
Antenna : 32m parabolic antenna
Gain: 41.0 dB
Figure of Merit: 7.46 MWm2K-1
Frequency Range: 1290 MHz ± 75 kHz(?)
Transmittter Type: Klystrons (modular Tx soon?)
Transmitter (Average) Power: 120 kW
Duty Cycle: 3%
Hours/Year: ~1200
Pulse Codes: Alternating, random codes, long pulses
Millstone Hill IS Radars, Massachusetts, USA
Millstone: Facts and Figures
Antennas: 46m steerable (MISA), 68m fixed (zenith)
Gain: 42.5 dB (MISA), 45.0 dB (zenith)
Figure of Merit: 8.3 (MISA), 14.7 (zenith) MWm2K-1
Frequency Range: 440.2 MHz ± 200 kHz (?)
Transmitter Type: Klystrons
Transmitter (Average) Power: 150 kW
Duty Cycle: 6%
Hours/Year: ~1500
Pulse Codes: Alternating, random codes, long pulses
Kharkov ISR, Ukraine (left)
Irkutsk ISR, Siberia (right)
Jicamarca ISR, Peru
Jicamarca: Facts and Figures
Antennas: 85,000 m2 phased array (18,432 dipoles)
Gain: 45.0 dB
Figure of Merit: 13.72 MWm2K-1
Frequency Range: 49.9 MHz ± 0.5 MHz
Transmitter Type: Klystrons + transmitting array
Transmitter (Average) Power: 120 kW
Duty Cycle: 6%
Hours/Year: >2000
Pulse Codes: Alternating, random codes, long pulses
MU Radar, Japan
MU Radar: Facts and Figures
Antennas: 103m diameter phased array (475 dipoles)
Gain: 34.0 dB
Figure of Merit: 0.52 MWm2K-1
Frequency Range: 46.5 MHz ± 0.8 MHz
Transmitter Type: Distributed transmitters in array
Transmitter (Average) Power: 50 kW
Duty Cycle: 4%
Hours/Year: < 1000 hours
Pulse Codes: Long pulse and multipulse
Advanced Modular Incoherent Scatter Radar
(AMISR)
AMISR (1 Face):
Facts and Figures
Antennas: 32m2 phased array face (=128 panels)
Gain: 41.0 dB
Figure of Merit: 10.29 MWm2K-1
Frequency Range: 430 MHz ± ??
Transmitter Type: Distributed within array
Transmitter (Average) Power: 210 kW
Duty Cycle: 10%
Hours/Year: Operational winter 2006
Pulse Codes: Alternating, random codes, long pulses
AMISR Test Panel at SRI
Where have we got so far?
So far, we have seen;
•that incoherent scatter radars come in all shapes and sizes !!
•how klystrons work
•what the beam pattern looks like for a parabolic dish
•why antennas have to be so big
•how to derive raw electron density from the received power
For any other parameters, (temperature, velocity, collision frequency) we need to
measure the shape of the spectrum (the distribution of power with respect to the
frequency).
In other words, it’s not enough just to detect the power of the received signal. We
also have to detect its spectral shape and Doppler shift
So how do we do that?